{"id":1391,"date":"2017-10-12T14:33:16","date_gmt":"2017-10-12T14:33:16","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/?post_type=chapter&p=1391"},"modified":"2017-10-12T14:33:16","modified_gmt":"2017-10-12T14:33:16","slug":"examples-that-show-how-delocalized-electrons-affect-stability","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/chapter\/examples-that-show-how-delocalized-electrons-affect-stability\/","title":{"raw":"Examples That Show How Delocalized Electrons Affect Stability","rendered":"Examples That Show How Delocalized Electrons Affect Stability"},"content":{"raw":"
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\u00a0In the vast majority of the nucleophilic substitution reactions you will see in this and other organic chemistry texts, the electrophilic atom is a carbon which is bonded to an electronegative atom, usually oxygen, nitrogen, sulfur, or a halogen. The concept of electrophilicity is relatively simple: an electron-poor atom is an attractive target for something that is electron-rich, i.e<\/em>. a nucleophile. However, we must also consider the effect of steric hindrance on electrophilicity. In addition, we must discuss how the nature of the electrophilic carbon, and more specifically the stability of a potential carbocationic intermediate, influences the SN<\/sub>1 vs. SN<\/sub>2 character of a nucleophilic substitution reaction.<\/div>\r\n<\/div>\r\n
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8.4A: Steric effects on electrophilicity<\/h3>\r\nConsider two hypothetical SN<\/sub>2 reactions: one in which the electrophile is a methyl carbon\u00a0 and another in which it is tertiary carbon.\r\n\r\n\"image048.png\"\r\n\r\nBecause the three substituents on the methyl carbon electrophile are tiny hydrogens, the nucleophile\u00a0 has a relatively clear path for backside attack.\u00a0 However, backside attack on the tertiary carbon is blocked by the bulkier methyl groups.\u00a0 Once again, steric hindrance\u00a0 - this time caused by bulky groups attached to the electrophile rather than to the nucleophile - hinders the progress of an associative nucleophilic (SN<\/sub>2) displacement.\r\n\r\nThe factors discussed in the above paragraph, however, do not prevent a sterically-hindered carbon from being a good electrophile - they only make it less likely to be attacked in a concerted SN<\/sub>2 reaction<\/em>.\u00a0 Nucleophilic substitution reactions in which the electrophilic carbon is sterically hindered are more likely to occur by a two-step, dissociative (SN<\/sub>1) mechanism. This makes perfect sense from a geometric point of view:\u00a0 the limitations imposed by sterics are significant mainly in an SN<\/sub>2 displacement, when the electrophile being attacked is a sp3<\/sup>-hybridized tetrahedral carbon with its relatively \u2018tight\u2019 angles of 109.4o<\/sup>.\u00a0 Remember that in an SN<\/sub>1 mechanism, the nucleophile attacks an sp2<\/sup>-hybridized carbocation intermediate, which has trigonal planar geometry with \u2018open\u2019 120 angles.\r\n\r\n\"image050.png\"\r\n\r\nWith this open geometry, the empty p orbital of the electrophilic carbocation is no longer significantly shielded from the approaching nucleophile by the bulky alkyl groups. A carbocation is a very potent electrophile, and the nucleophilic step occurs very rapidly compared to the first (ionization) step.\r\n\r\n<\/div>\r\n
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8.4B: Stability of carbocation intermediates<\/h3>\r\nWe know that the rate-limiting step of an SN<\/sub>1 reaction is the first step - formation of the this carbocation intermediate.\u00a0 The rate of this step \u2013 and therefore, the rate of the overall substitution reaction \u2013 depends on the activation energy for the process in which the bond between the carbon and the leaving group breaks and a carbocation forms.\u00a0 According to Hammond\u2019s postulate (section 6.2B<\/a>), the more stable the carbocation intermediate is, the faster this first bond-breaking step will occur. In other words, the likelihood of a nucleophilic substitution reaction proceeding by a dissociative (SN<\/sub>1) mechanism depends to a large degree on the stability of the carbocation intermediate that forms.\r\n\r\nThe critical question now becomes, what stabilizes a carbocation<\/em>?\r\n\r\nThink back to Chapter 7<\/a>, when we were learning how to evaluate the strength of an acid.\u00a0 The critical question was \u201chow stable is the conjugate base that results when this acid donates its proton\"?\u00a0 In many cases, this conjugate base was an anion \u2013 a center of excess electron density.\u00a0 Anything that can draw some of this electron density away\u2013 in other words, any electron withdrawing group \u2013 will stabilize the anion.\r\n\r\nSo if it takes an electron withdrawing<\/em> group to stabilize a negative charge, what will stabilize a positive charge?\u00a0 An electron donating<\/em> group!\r\n\r\n\"image052.png\"\r\n\r\nA positively charged species such as a carbocation is very electron-poor, and thus anything which donates electron density to the center of electron poverty will help to stabilize it. Conversely, a carbocation will be destabilized<\/em> by an electron withdrawing group.\r\n\r\nAlkyl groups \u2013 methyl, ethyl, and the like \u2013 are weak electron donating groups, and thus stabilize nearby carbocations.\u00a0\u00a0 What this means is that, in general, more substituted carbocations are more stable<\/em>:\u00a0 a tert-butyl carbocation, for example, is more stable than an isopropyl carbocation.\u00a0 Primary carbocations are highly unstable and not often observed as reaction intermediates; methyl carbocations are even less stable.\r\n\r\n\"image054.png\"\r\n\r\nAlkyl groups are electron donating and carbocation-stabilizing because the electrons around the neighboring carbons are drawn towards the nearby positive charge, thus slightly reducing the electron poverty of the positively-charged carbon.\r\n\r\nIt is not accurate to say, however, that carbocations with higher substitution are always<\/em> more stable than those with less substitution. Just as electron-donating groups can stabilize a carbocation, electron-withdrawing groups act to destabilize carbocations. Carbonyl groups are electron-withdrawing by inductive effects, due to the polarity of the C=O double bond.\u00a0\u00a0 It is possible to demonstrate in the laboratory (see section 16.1D<\/a>) that carbocation A below is more stable than carbocation B, even though A is a primary carbocation and B is secondary.\r\n\r\n\"image056.png\"\r\n\r\nThe difference in stability can be explained by considering the electron-withdrawing inductive effect of the ester carbonyl. Recall that inductive effects - whether electron-withdrawing or donating - are relayed through covalent bonds and that the strength of the effect decreases rapidly as the number of intermediary bonds increases.\u00a0 In other words, the effect decreases with distance.\u00a0 In species B the positive charge is closer to the carbonyl group, thus the destabilizing electron-withdrawing effect is stronger than it is in species A.\r\n\r\nIn the next chapter we will see how the carbocation-destabilizing effect of electron-withdrawing fluorine substituents can\u00a0 be\u00a0 used in experiments designed to address the question of whether a biochemical nucleophilic substitution reaction is SN<\/sub>1 or SN<\/sub>2.\r\n\r\nStabilization of a carbocation can also occur through resonance effects, and as we have already discussed in the acid-base chapter, resonance effects as a rule are more powerful than inductive effects.\u00a0 Consider the simple case of a benzylic <\/strong>carbocation:\r\n\r\n\"image058.png\"\r\n\r\nThis carbocation is comparatively stable.\u00a0 In this case, electron donation is a resonance effect.\u00a0 Three additional resonance structures can be drawn for this carbocation in which the positive charge is located on one of three aromatic carbons. The positive charge is not isolated on the benzylic carbon, rather it is delocalized around the aromatic structure: this delocalization of charge results in significant stabilization.\u00a0 As a result, benzylic and allylic<\/strong> carbocations (where the positively charged carbon is conjugated to one or more non-aromatic double bonds) are significantly more stable than even tertiary alkyl carbocations.\r\n\r\n\"image060.png\"\r\n\r\nBecause heteroatoms such as oxygen and nitrogen are more electronegative than carbon, you might expect that they would by definition be electron withdrawing groups that destabilize carbocations.\u00a0 In fact, the opposite is often true: if the oxygen or nitrogen atom is in the correct position, the overall effect is carbocation stabilization.\u00a0 This is due to the fact that although these heteroatoms are electron withdrawing<\/em> groups by induction, they are electron donating<\/em> groups by resonance, and it is this resonance effect which is more powerful.\u00a0\u00a0 (We previously encountered this same idea when considering the relative acidity and basicity of phenols and aromatic amines in section 7.4<\/a>).\u00a0 Consider the two pairs of carbocation species below:\r\n\r\n\"image062.png\"\r\n\r\n\"image064.png\"\r\n\r\nIn the more stable carbocations, the heteroatom acts as an electron donating group by resonance: in effect, the lone pair on the heteroatom is available to delocalize the positive charge.\u00a0 In the less stable carbocations the positively-charged carbon is more than one bond away from the heteroatom, and thus no resonance effects are possible.\u00a0 In fact, in these carbocation species the heteroatoms actually destabilize <\/em>the positive charge, because they are electron withdrawing by induction.\r\n\r\nFinally, vinylic<\/strong> carbocations, in which the positive charge resides on a double-bonded carbon, are very unstable and thus unlikely to form as intermediates in any reaction.\r\n\r\n\"image066.png\"\r\n
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Examples<\/h3>\r\n\"image068.png\"\r\n
Solution<\/a><\/div>\r\nIn which of the structures below is the carbocation expected to be more stable? Explain.<\/div>\r\n<\/div>\r\nFor the most part, carbocations are very high-energy, transient intermediate species in organic reactions. However, there are some unusual examples of very stable carbocations that take the form of organic salts. Crystal violet is the common name for the chloride salt of\u00a0 the carbocation whose structure is shown below.\u00a0 Notice the structural possibilities for extensive resonance delocalization of the positive charge, and the presence of three electron-donating amine groups.\r\n\r\n\"image070.png\"\r\n
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Example<\/h3>\r\nDraw a resonance structure of the crystal violet cation in which the positive charge is delocalized to one of the nitrogen atoms.\r\n\r\nSolution<\/a>\r\n\r\n<\/div>\r\n<\/div>\r\nWhen considering the possibility that a nucleophilic substitution reaction proceeds via<\/em> an SN<\/sub>1 pathway, it is critical to evaluate the stability of the hypothetical carbocation intermediate.\u00a0 If this intermediate is not sufficiently stable, an SN<\/sub>1 mechanism must be considered unlikely, and the reaction probably proceeds by an SN<\/sub>2 mechanism.\u00a0 In the next chapter we will see several examples of biologically important SN<\/sub>1 reactions in which the positively charged intermediate is stabilized\u00a0 by inductive and resonance effects inherent in its own molecular structure.\r\n
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Example<\/h3>\r\nState which carbocation in each pair below is more stable, or if they are expected to be approximately equal. Explain your reasoning.\r\n\r\n\"image072.png\"\r\n
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Solution<\/a><\/div>\r\n<\/div>\r\n\r\n\r\n<\/div>\r\n
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Contributors<\/h3>\r\n